This invention has to do with propellants for use in gun systems. More specifically, this invention has to do with a novel shape for propellant grains which enhances the burning characteristics of the propellant material such as to achieve higher muzzle velocity of a projectile without increasing the maximum experienced pressure within the barrel of the gun in use.
As is well known, the purpose of propellant materials in a gun system is to provide a source of energy for accelerating a projectile within the bore of a gun so that a desired muzzle velocity for the projectile is achieved. The projectile, initially at rest, is accelerated by force resulting from the generation of high pressure gaseous products in response to the ignition and burning of the propellant material.
As is also generally known, the burning of solid propellants ordinarily utilized in gun systems is initiated by some action, e.g. the release of a firing pin, which generates a small amount of hot gas in proximity to the propellant thus causing the propellant material to ignite and the burn process to commence.
Once ignition is achieved, it is desirable to have the propellant burn in a controlled manner from the surface of the propellant grains inwardly. Where the burn is essentially uniform over the whole propellant grain, the surface receeds parallel to itself, gas is generated evenly and the resulting pressure accelerates the projectile down the bore.
As is also well known in the arts, the ultimate or muzzle velocity of a projectile thus accelerated is related to and dependent upon the pressure-time history after ignition. Thus maximum pressure achieved during the burn as well as the magnitude of the sustained pressure after maximum has been reached are the primary factors in being able to achieve desired muzzle velocity within the limitations of acceptable gun structure.
For purposes of understanding fully the present invention, it is considered to be worthwhile to review the relationships of various factors which have an effect on muzzle velocity in any particular gun system.
The differential equations governing the acceleration of a projectile down the bore of a gun to a desired velocity are Newton's Law and the Propellant Burning Law respectively: ##EQU1## where: P=Pressure acting on base of projectile
A=bore area PA1 f=engraving and frictional force PA1 m=mass of projectile PA1 x=projectile travel ##EQU2## S=surface area of solid propellant (dr/dt)=propellant rate of surface regression PA1 t=time PA1 t.sub.m =time to travel to forward end or muzzle, of gun.
The launch velocity, from equation (1) Newton's Law is: ##EQU3## where: v=velocity
Because the friction factor in equation (1) i.e. the equation for Newton's law, is a small constant, it is clear that muzzle velocity is essentially proportional to the integral of the pressure-time history for a projectile starting from rest. Clearly, therefore, muzzle velocity can be increased by increasing the maximum pressure. However, as is discussed below, increasing maximum pressure is not an acceptable approach in many instances because of inherent limitation in presently known gun structures as well as because of the resultant fatigue stresses which shorten gun life. Further, increased maximum pressures are known to cause damage to projectiles some time with catastrophic failure of the weapon. Thus, improvement of muzzle velocity by increasing the maximum pressure during acceleration is not the most desirable approach to the problem.
Considering therefore the Propellant Burning Law, see equation (2) it can be seen that this law relates to the rate of gas evolution of the burning propellant material. The pressure time history generated thereby is the result of comparing the rate of pressure generation as a result of propellant burn with the increase in the volume of the gun chamber resulting from projectile displacement. As the propellant is initially ignited and gases are being generated, the projectile is either at rest or moving relatively slowly. Thus, gases are being generated faster than the volume of the chamber is increasing. Clearly as a result of this, the pressure experienced increases.
As the projectile accelerates down the gun bore, the volume of the chamber increases at a rate which ultimately surpasses the rate of gas generation by the burning of the propellant material. The transition corresponds to the point of maximum pressure in the chamber. Thereafter the pressure decreases as the projectile continues to accelerate thus increasing the volume of the gun chamber at a rate faster than the increase in volume of gases being generated by the propellant burn.
It has been recognized by those skilled in these arts that the rate of burn of propellant material i.e. the burn characteristics of the grain, is a function not only of the physical and chemical characteristics of the material itself but also of the shape of the grain. Known grain designs are ordinarily cylindrical elements having a single perforation therethrough or seven perforations therethrough. It has been found that grain designs having these characteristics are limited in their capability for extending of a relatively high degree of chamber pressure after the maximum pressure has been achieved. Thus, increases in muzzle velocities have been required to be achieved by increasing the maximum pressure in the gun system. However, as will be recognized by those skilled in these arts, such increases in maximum pressure are extremely expensive and result in difficult operational problems because of the requirement for increased structural capabilities of the cannon, rolling stock, support stock, and the like.
Prior attempts to achieve a higher sustained pressure subsequent to the achievement of maximum pressure have not been successful nor, for economic reasons, has it been found acceptable to resort to more esoteric propellant materials.
Typical prior art approaches may be seen by way of example in U.S. Pat. No. 4,094,248; U.S. Pat. No. 3,429,624; British Pat. No. 7178 and French Pat. No. 1,595,508.
In U.S. Pat. No. 4,094,248 there is described a hexoganol grain with 7 internal perforations centered on the vertices of equilateral triangles, with each grain having external longitudinal grooves. This geometry does not provide for progressive burning and performance characteristics. Further, there exists gaps between the faces of adjacent propellant grains which would permit burning to take place on the face as well as the external grooves such that the actual performance of the grain design as disclosed in the patent will be similar to that of the 7 perforation cylindrical grains which are the standard U.S. gun propellant geometry.
U.S. Pat. No. 3,429,264 described a solid stick propellant with a grooved hexoganol-like periphery which can be used in place of tubular propellant in rockets. There is no provision for perforations. Further, the patent teaches degressive burning, i.e. rapid initial burning and slower burning in final stages. This concept is diametrically opposite to that concept disclosed in the present application which provides for progressive burning which is essential to improved gun performance.
French Pat. No. 1,595,508 describes a block of propellant formed by bonding individual propellant grains together in a matrix. The progressive burning achieved pursuant to this approach is the result of burning on the external surfaces as being inhibited by the bonding agent.
British Pat. No. 7178 describes perforated propellants, perforations of which are of a shape other than cylindrical. Further, in order to maintain the equal distance, the British Patent teaches the use of grooves on the periphery to maintain the same web throughout the grain.